Think of them as QantasLink but without being a member of oneworld.
Malaysian fully owns two regional airlines - Maswings which serves the Western part (ie Sabah, Borneo) and Firefly on the main island.
While I haven't flown Firefly, Maswings is very good - even has business class in its ATRs - the back two rows (closest to the entry/exit point) and the short flight services include snacks, and a curtain divider.
???? Not to USM, neither fly there, assume you must be talking about SIN/KUL. From memory the biggest aircraft into USM are 734 (TG) and 319 (MI, PG). Assume A320's are too big for TR or 3K (or Air Asia) to fly there, either that or landing fees to expensive.
For KL to Koh Samui, options are pretty limited anyway. It's good that Firefly operate from Subang, much closer to the city centre than KLIA. Bangkok Airways has flights from KLIA, and can be cheaper than Firefly.
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I've done the reverse of that several times and my experience has been same. subang is a LCC terminal and very basic. But otherwise no issues. Even used their call center who spoke good English and were helpful.
I flew FY from PEN to SZB and the only memorable thing for me was a slight delay in departure as all passengers were already seated on board but then we had to wait for the aircraft to refuel while it was parked on the tarmac under the hot sun without any air conditioning. The next sector SZB to SIN was uneventful.